Saturday, October 22, 2011

Saturday, October 1, 2011

Vacuum...................

....tube amplifiers. The top image is a Bell & Howell projector audio amplifier. Lucked out in a surplus find. Dismanted the unit and sprayed liberally with DeOxit. Applied power and She Lives! The volume pot was scratchy -- but another liberal dousing of DeOxit fixed that issue. This unit needs no pre-amp, has volume and a multi-stage tone control with external speaker options. I think it'll make a great guitar amp with little modification.

The working side of the Bell & Howelll amplifier. Nice and clean.

This puppy is a tube amplifer from a 60's Capitol record player. I was asked to fix it by one of the baristas at the coffee shop I frequent. After much GOOGLE searching, I determined the 25L6 tube heaters had been powered in series with a (now missing) motor. Since the 25L6 heaters are 25v, this necessitated a 90v drop at 300 mA...but no matter what I did, I could not get the tube to fire up. Finally applied 24v to the tube heaters and found the tube was dead. Dug through my stash of old 'dead' tubes until I found some 50L6s -- same pinout, but with a 50v 150 mA heater. After a bit of trial and error, I found a 1.2K and 690 ohm 25w resistor in parallel gives me 49 volts at the heaters in operation...and the amplifer works. I very much doubt it's really suited as a guitar amp -- it definitely needs a pre-amp and the overall wattage is very low. The real issue with an old tube amp like this is that it's VERY unsafe -- there is direct connection to 120 without a transformer - so if the capacitor fails....the chassis becomes hot. Or..if the wall socket is wired wrong (I did attach a 3-prong plug to it)...the the chassis becomes hot. So...quite unsafe, but interesting to tinker with.

This has been my first foray into vacuum tube electronics since the 70s and quite a learning experience....but diverting and amusing.